Cannabis has some medical benefits, but may not be the panacea that its advocates claim.

Researchers have looked at trials using three different cannabinoids - extracts from the drug - to see how they work in different circumstances.

Results are mixed and cannabinoids were found to be better than conventional drugs for treating sickness caused by chemotherapy.

But they were no more effective than codeine for controlling acute pain relief.

I would like to be cautiously optimistic that, with adequate research, cannabinoids will be effective for chronic pain

Dr Fiona Campbell,Pain Management Centre, Nottingham

Dr Fiona Campbell, of Nottingham's Pain Management Institute, was part of the research team.

She believes the key area for future research may be chronic pain, such as that caused by multiple sclerosis spasms, for which there are no effective treatments currently available.

Earlier this year, it was announced the first clinical trials of cannabis-based medicines involving patients suffering from MS, spinal cord injury and other forms of severe pain would be carried out by GW Pharmaceuticals

Dr Philip Robson, the company's medical director, said GW had carried out clinical trials with 75 patients over the last 18 months, and that improvements had been seen in a wide range of symptoms.

"Patients in these trials are clearly gaining benefit. In some cases, the improvement has been sufficient to transform lives."

The trials use medicines derived from whole extracts of varieties of cannabis.

Patients in these trials are clearly gaining benefit. In some cases, the improvement has been sufficient to transform lives

Dr Philip Robson

But the researchers, from Nottingham and Hopitaux Universitaires, Geneva, Switzerland say widespread use of cannabinoids in treatment is unlikely because they have too many side-effects.

The two studies, published in the British Medical Journal, analysed trials carried out between 1975 and 1997 into the pain relief and sickness prevention qualities of cannabinoids.

The pain relief study looked at results from nine trials involving over 200 patients, five for cancer pain, two for chronic non-malignant pain and two for postoperative pain.

In eight of the nine trials, cannabinoids were no more effective than the everyday painkiller codeine.

The scientists suggest cannabinoids may be useful in long-term chronic pain associated with conditions other than cancer, but that it is not useful in treating the acute pain people feel after operations.

In the second study, 30 trials covering 1,300 patients and checking the use of cannabinoids for controlling nausea and sickness caused by chemotherapy were analysed.

Side-effects

Researchers found cannabinoids seemed to be more effective than conventional treatments.

There were some harmful side-effects found with cannabinoids, although others found the drugs had beneficial side-effects such as euphoria and drowsiness.

These findings lead the researchers to suggest cannabinoids could be useful as mood-enhancing drugs for controlling sickness caused by chemotherapy.

The body has cannabinoid receptors in the nervous system.

Tests on animals have given some hope that cannabinoids could work to reduce pain.

Dr Campbell added: "If we have anything that can help relieve pain, it should be evaluated.

Smoking the key

"I would like to be cautiously optimistic that, with adequate research, cannabinoids will be effective for chronic pain."

Cannabis campaigners say it is smoking the drug which gives the benefits, not taking one of the cannabinoids.

Lesley Gibson of the UK Cannabis Internet Activists group is an MS sufferer who successfully used the defence of medicinal use against charges of possession of cannabis last year.

She said scientists should evaluate the drug as a whole, not extracts which did not work on their own.

"There are that many cannabinoids in cannabis, you can't isolate it. It works as a whole," she said.